1. Technical Field
The present disclosure relates to master oscillators, laser systems, and laser light generation methods.
2. Related Art
Typical ultraviolet excimer lasers used in semiconductor lithography processes include a KrF excimer laser having a wavelength of approximately 248 nm and an ArF excimer laser having a wavelength of approximately 193 nm.
Most such ArF excimer lasers are supplied to market as two-stage laser systems that include an oscillation stage laser and an amplifier stage. A basic configuration that is common between the oscillation stage laser and the amplifier stage in a two-stage ArF excimer laser system will be described. The oscillation stage laser has a first chamber, whereas the amplifier stage has a second chamber. A laser gas (a mixed gas including F2, Ar, Ne, and Xe) is confined in the respective first and second chambers. The oscillation stage laser and the amplifier stage also have power sources that supply electrical energy for pumping the laser gas. Separate power source may be provided for the oscillation stage laser and the amplifier stage, respectively, but a single power source can also be shared between the two. First discharge electrodes including a first anode and a first cathode that are both connected to the power source are provided within the first chamber. Similarly, second discharge electrodes including a second anode and a second cathode that are both connected to the power source are provided within the second chamber.
A configuration unique to the oscillation stage laser is, for example, a line narrowing module. A line narrowing module typically includes a single grating and at least one prism beam expander. An optical resonator is formed jointly by a semitransparent mirror and the grating, and the first chamber of the oscillation stage laser is disposed between the semitransparent mirror and the grating.
When a discharge is generated between the first anode and the first cathode of the first discharge electrodes, the laser gas is pumped, and light is generated when the pumping energy is emitted. This light results in laser light whose wavelength has been selected by the line narrowing module, and the laser light is outputted from the oscillation stage laser.
A two-stage laser system in which the amplifier stage is a laser including a resonator structure is called “MOPO,” whereas a two-stage laser system in which the amplifier stage is not a laser and no resonator structure is provided is called “MOPA.” When the laser light from the oscillation stage laser is present within the second chamber of the amplifier stage, control is carried out so that a discharge is created between the second anode and the second cathode of the second discharge electrodes. Through this, the laser gas within the second chamber is pumped, and the laser light is outputted from the amplifier stage after being amplified.